Electroanalytical Investigation of the Electrode-Electrolyte Interface of Quaternary Ammonium Ionic Liquids: Impact of Alkyl Chain Length and Ether Functionality

摘要

The influence of ionic associations and potential-dependent interactions on the electrode-electrolyte interfacial structure of ionic liquids (ILs) is studied by electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS) for a variety of asymmetric quaternary ammonium ILs. Specifically, the impact of cation alkyl chain length (C = 4, 8 and 16) and ether functionality on the interfacial structuring of ILs at the glassy carbon electrode surface is examined. Ammonium cations with alkyl chain length of 8 and 16 carbons are found to stabilize the formation of the bis(trifluorosulfonyl)imide, [TFSI], anion dense Stern layer at positive electrode potentials leading to larger capacitances. The longer alkyl chain of the cation is believed to screen the ion-ion repulsion among the anions by intruding into the interfacial anion layer. SERS suggests the presence of carbon-containing rings at the interface at both positive and negative electrode potentials, which can be explained by the buckling of the long alkyl chains. Inclusion of an ether functionality allowed for more symmetry in the camel-shaped potential-dependent differential capacitance curves, suggesting similar excess ion density at both positive and negative potentials. This work contributes to understanding and predicting the interfacial electrode capacitance in ILs by understanding the balance of ionic interactions and the associated repulsions at electrode-electrolyte interfaces that are pertinent to electrochemical energy storage, electrocatalysis, and electrochemical sensors.